Good Bugs Gone Bad

Introduced into this country to control pests, these insects now wreak havoc of their own

08-01-2005
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Doug Stewart

WHEN A PLAGUE of tree-climbing aphids afflicted pecan orchards in the southeastern United States in the 1970s, federal biologists released a tree-climbing ladybug from Asia to devour them. The multicolored Asian lady beetle (Harmonia axyridis) did a superb job. Unfortunately, it also revealed two unwelcome traits: the wanderlust of a hobo and the appetite of a gourmand. Swarms of the little beetles eventually marched up the Atlantic seaboard into New England and westward across the Mississippi. Along with another foreign import, the seven-spotted ladybug (Coccinella septempunctata) brought over earlier from Europe, the ravenous Asian insect has eaten so many aphids so fast that many native ladybugs may have been left with too little to eat. To make matters worse, the newcomers are apt to eat the hometown ladybugs, too. Possibly due to this onslaught, even New York’s official state insect, the nine-spotted lady beetle (Coccinella novemnotata), is now extinct in the state.

In the field of classical biological control—the use of exotic natural enemies to counter invasive pests—examples of biocontrol insects that have themselves gone out of control are relatively few. When it works, biological control is more benign, more efficient, and more precisely targeted than the usual method of controlling pest species, which is spraying their general whereabouts with toxic chemicals.

But just as an exotic plant can turn invasive when freed from the enemies that kept it in check back home, so too can an exotic biocontrol insect run amuck itself in the absence of the predators and competitors it evolved with. (The organisms used for biocontrol include not only insects but also other arthropods, viruses, fungi, bacteria, nematodes and other life-forms.) And when good bugs go bad, they can make big trouble. Whereas a chemical pesticide weakens over time, living creatures have a way of multiplying.

“With a chemical spill, you could theoretically recover all the molecules,” says entomologist May Berenbaum of the University of Illinois. “With a biological organism that reproduces, once it’s out there, it’s out there. You can’t round it up.”

The first successful U.S. deployment of a biocontrol insect occurred more than a century ago. An Australian ladybug, the vedalia beetle (Rodolia cardinalis), was released in California citrus orchards in 1888 to combat cottony cushion scale. In decimating the citrus pest and keeping it in check ever since, without collateral damage or becoming a pest itself, the beetle has performed superbly—possibly a little too superbly, suggests Erin Stephens, a Cornell University insect ecologist. “It led people to think they could use biocontrol agents as a cure-all,” she says, “when in fact they just got really, really lucky.”

It was common before 1950 (and not uncommon into the 1980s) to look for pest-fighters with broad appetites. Today, biological-control experts enlist only those insects that attack an extremely narrow range of enemies, ideally a single invasive species. Before the U.S. Department of Agriculture approves a foreign insect for release to control a pest plant, its host preferences are methodically tested under tight quarantine conditions. Not surprisingly, the vast majority of good bugs gone bad are insects that were released long before this kind of policing became common.

On the Hawaiian island of Kauai prior to 1950, parasitic wasps from China and the U.S. mainland were released at least 100 times by the Hawaiian Sugar Planters’ Association to fight sugar-cane pests. (Parasitic insects lay eggs inside the larvae of other insects, the hatchlings killing their hosts from within.) In 1999 and 2000, ecologists Jane Memmott and Laurie Henneman of England’s University of Bristol collected more than 2,000 caterpillars of native moths from the island’s Alakai Swamp. High, remote and inhospitable, the swamp is one of the wettest places on Earth, bearing little resemblance to the warmer, drier sugar-cane plantations near the coast. Nonetheless, the researchers found exotic wasps developing from eggs in one in five native swamp caterpillars. And most of these, they discovered, were from just three species of parasitic wasps deliberately released in the cane fields more than a half century ago.

That the alien wasps weren’t picky about either habitat or host was probably fine with the people who brought them to Hawaii, says Memmott. “What people cared about then were crops, not native insects. If they were conservationists, it was rare Hawaiian birds they were interested in, not little moths.” On Kauai as elsewhere, she says, ignoring native insects was misguided. “Invertebrates are the powerhouse that keeps everything else going. Without those native caterpillars, there wouldn’t be any insect-eating birds.”

Some moths are more obviously worth protecting. The once-common cecropia moth (Hyalophora cecropia), for example, is one of the largest and most spectacular insects in North America. It is now becoming rare in the Northeast, and a parasitic fly that was supposed to control gypsy moths is apparently the culprit. From 1906 to 1986, Compsilura concinnata, a European fly, was released repeatedly in North America by both the U.S. and Canadian governments as a biocontrol agent. C. concinnata has four generations from May to October, according to George Boettner of the University of Massachusetts, whereas gypsy moth caterpillars are only out in May and June. Says Boettner, “This means Compsilura must be hitting other things as well.” A lot of other things. Boettner calculated the number of flies produced on gypsy moths in New Jersey in the 1980s and found that “on average, every two-and-a-half acre plot of forest was producing enough flies to potentially kill 750,000 moths and butterflies in the next generation, per year.”

To figure out what all those flies were attacking after gypsy moths pupated, Boettner and his colleagues Joe Elkinton and Cynthia Boettner checked a list of locally declining butterflies and moths to see which ones hatched a few months after gypsy moths did. That led them to silk moths. Sure enough, when they set out cecropia and other silk moth caterpillars for just a week, four out of five were parasitized by C. concinnata.

“Out of 500 cecropia caterpillars we put out on trees, none survived to make it to pupae. None even made it past 40 days of their 60-day life span as caterpillars. It was crazy.” C. concinnata has now been found in some 200 species of moths and butterflies, many of which are in decline. On Boettner’s to-do list: find whatever it is in Europe that keeps C. concinnata under control there.

The most commonly used biological-control insects are herbivores, which control invasive weeds by feeding on them. Here, too, picky eating habits are essential. You don’t want a bug that’s supposed to kill kudzu laying waste to forested wilderness, let alone the American soybean crop. Despite the risks, the consensus among experts on invasive weeds is that properly managed biocontrol programs are invaluable. In vast natural areas such as the Everglades, where spraying, snipping or bulldozing exotic weeds is out of the question, a well-tested, well-behaved bug is the conservationist’s best weed whacker.

Sometimes a bug’s narrow appetite isn’t quite narrow enough, however. When a foreign weed invades territory occupied by closely related native plants, a biocontrol insect may fail to tell which is which. In 1989, the Argentine cactus moth (Cactoblastis cactorum), used since the 1950s to control invasive cacti in the Caribbean, turned up in Florida, probably having stowed away in a shipment of houseplants.

“There are five native prickly pear cacti in Florida, and the moth uses all of them as hosts,” says Robert Pemberton, a research entomologist at the USDA’s Invasive Plant Research Lab in Fort Lauderdale. “It has now spread up to the Florida panhandle and into Alabama. Our fear is that it will spread around the Gulf of Mexico to Texas, where there are nearly 100 species of native cacti.” Some of these plants are rare and could be swamped by the large numbers of moths generated by the more common prickly pear species. No doubt there are ranchers who would be happy to see cacti vanish from their rangeland, but native dry-land cacti have an important environmental role, Pemberton points out. They’re used as food for wildlife and, in Mexico (where the moth could head next), for people.

Thistles, like cacti, are underappreciated plants, perhaps because of their spikiness. This may help explain why native thistles in North America were exposed to the depredations of a thistle-eating weevil from Europe. The flower head weevil, Rhinocyllus conicus, was released by the Canadian Ministry of Agriculture into Canada in the late 1960s and then by the U.S. Department of Agriculture into Virginia, Nebraska, Montana, California and other states. The idea was to control invasive weeds, especially Eurasian musk thistle, though R. conicus was known to feed on native rangeland thistles, too.

Svata Louda, a University of Nebraska ecologist who studies native thistles and the organisms that keep them in check, first noticed R. conicus in the flower heads of native thistles in 1993. “In 1994, the weevil had doubled its numbers. By the next year, it had quadrupled,” she says. “It was increasing at an exponential rate on the native species because there were no exotic thistles around for it to eat.” She worries that as the weevil continues to expand its range, it is likely to drive several species of native thistle extinct, including the threatened pitcher’s thistle (Cirsium pitcheri) that clings to survival in the dunes along the Great Lakes.

Despite their spiky exteriors, native thistles are worth caring about, Louda says. The American goldfinch lines its nest with their down and feeds on their large seeds, while bees and other insects, including an endangered Northeastern fritillary butterfly, rely on their nectar-rich flowers. There’s also a less obvious benefit of these native plants. One of the country’s most aggressive exotic thistles is bull thistle, an invasive pest in many western states. In Nebraska, it’s not a problem. Louda decided to find out why. “We discovered that native insects feed on a native thistle here in Nebraska that flowers at the same time as this exotic species, so they move right over and feed on the exotic thistle, too.” Thanks to the native plant’s similar timing and palatability, the alien plant has never gotten more than a toehold in the state.

The interplay among biocontrol insects, their intended targets and various innocent bystanders (including us) can be baffling in their complexity. European seed-head flies released since the early 1970s to control spotted knapweed, a nationwide scourge, can produce larvae in such abundance that larvae-eating deer mouse populations sometimes soar in response. With a jump in deer mice comes a heightened risk of mouse-borne diseases, notably hanta virus. Ultimately, it’s a human choice: Which is worse, the weed or the illness?

“It’s a balancing act,” says Eric Coombs, biological control entomologist for the state of Oregon. “Is the good a biocontrol insect does outweighed by the bad? Right now, our tolerance level for bad is really, really low.”

Massachusetts writer Doug Stewart is a frequent contributor to this magazine.

America's Least Wanted

IUCN–The World Conservation Union recently released a list titled 100 of the World’s Worst Invasive Alien Species. Among the invertebrates included are these 14 insects, all found in the United States:

According to NWF Chief Naturalist Craig Tufts, one of the best ways to help contain populations of pest species is “to support diverse natural habitats” where native predators, such as ladybugs, soldier beetles and tachinid flies, can flourish. Following are some tips for creating an insectary in your yard—a place attractive to beneficial bugs that dwell naturally in your local area:

Grow native flowering plants, such as bee balm or coreopsis, that provide nectar and pollen to beneficials passing by, enticing them to stay.

Fill a shallow birdbath or dish with water and a few pebbles, giving pest fighters a place to sit and slurp.

Provide shelter, in the form of ground cover or a hedgerow, where insects can live and reproduce. Allowing a corner of your yard to grow wild—local ordinances permitting—is another way to help ensure insect diversity.

Eliminate the use of toxic pesticides, which generally fail to discriminate between harmful and helpful insects. Ideally, if you give beneficial bugs food, water, shelter and places to raise young, you won’t need to apply harmful chemical treatments.

When tamarisk was first brought to the United States from the shores of the Mediterranean in the 1800s, gardeners snapped up this ornamental tree for its feathery, needlelike leaves and dainty pink flowers. Its deep roots also made it a seemingly ideal plant for riverbank stabilization. Today, however, tamarisk, or salt cedar as it's often called, is an ecological nightmare. It blankets 1.2 million acres of the arid Southwest, where it is sucking up excessive amounts of water, pushing out natives and altering riparian or streamside habitat.

What makes a good plant go bad? In many parts of the world, tamarisk is a treasured tree. But when introduced into a new environment, tamarisk became invasive, growing rapidly and outcompeting its neighbors. This frequently happens when a plant is taken out of its native habitat, because it leaves behind the complex array of natural controls that held it in check.

About 1,100 plant species are now considered to be invasive in the United States, according to the Plant Conservation Alliance's Alien Plant Working Group. Some of these plants—such as tamarisk, purple loosestrife, and Norway maple—can drastically change their new environment. When this happens, not only do native plants lose out, but so do birds and other wildlife.

In one study along the lower Colorado River, for example, biologists discovered tamarisk groves supported less than ten percent of the winter bird life found in nearby stands of native trees. When purple loosestrife—a Eurasian aquatic—takes over a Midwestern marsh, it chokes out native rushes and sedges, causing ducks and other waterfowl to lose both food and nesting habitat. Norway maples, which are spreading along streams in the Appalachians and the Northern Rockies, produce a chemical that inhibits the growth of any other type of plant beneath their canopy.

Ornamentals that are harmful to our environment are still sold in many garden centers. Some of these plants, such as Japanese barberry, are even widely promoted by the nursery trade. This Asian exotic, which is a thorny deciduous shrub, has now escaped into the wild from Maine to Georgia and as far west as Wyoming. When unchecked by gardeners, Japanese barberry forms impenetrable thickets that exclude nearly all native plants and also create a physical barrier to wildlife.

The financial cost of these alien plants is staggering. In the United States, for example, over $100 million a year is spent fighting aquatic invasives. Our national parks also expend precious money and resources to keep nonnatives from taking over. In Great Smoky Mountains National Park, one of the most aggressive species is the mimosa, or silk tree. During the last decade, workers have cut down thousands of these flowering trees from just one river drainage.

Unfortunately, it's often difficult to predict ahead of time which exotic plants will turn into outlaws. The best course of action: Garden as much as possible with trees, shrubs, perennials and vines that are native to your area. If you do decide to plant nonnatives, studies have shown that hybrids rarely become invasive. Another option is to use species from different regions of the United States. More than 95 percent of North America's invasive plants have been brought here from other countries.

Whatever you do, give careful thought to what you are putting in your backyard habitat. By checking to see whether a species has the potential to become invasive before you plant it, you can do your part to protect local wildlife as well as America's biological heritage.—Doreen Cubie